1. Field of the Invention
The present invention relates to high intensity discharge lamps of the metal halide type and, in particular, to a high intensity discharge lamp containing a combination of metal halides including a particular range of concentrations of thorium iodide to control thorium transport to the electrode tips within the arc tube of the lamp to improve arc tube performance.
2. Description of the Prior Art
Metal halide arc discharge lamps utilize the addition of halides of various light emitting metals to mercury in the arc tube of a high pressure discharge lamp in order to modify the color temperature of the light emitted by the lamp and to raise the operating efficacy of the light output by the lamp as described in U.S. Pat. No. 3,234,421 issued Feb. 8, 1966 to Reiling and assigned to the assignee of the present invention. The electrodes of prior art metal halide lamps typically comprise a thorium tungsten electrode formed by wrapping a tungsten coil to serve as a heat radiator around a tungsten rod with a thorium or thorium oxide containing compound in the turns of the coil. Under proper conditions when operating in a thorium iodide containing atmosphere the tungsten rod acquires a thorium spot on its distal end which serves as a good electron emitter and which is continually renewed by a transport cycle involving the halogen present in the atmosphere which returns to the cathode any thorium lost by any process. The thorium tungsten electrode and its method of operation are described in Electric Discharge Lamps by John F. Waymouth, MIT Press, 1971 Chapter 9. Waymouth describes an analysis of the operation of the lamp electrodes in the presence of thorium as producing a coating on the tip of the lamp electrodes. When the electrode is not coated by thorium, the work function of the electrode is higher due to exposure of the tungsten and the resultant direct contact between the arc and the tungsten requiring the electrode to run hotter in order to sustain the arc current which causes poorer lumen maintenance especially in smaller sized lamps. The higher temperature makes the arc tube blacken due to tungsten loss from the electrodes which results in poor lumen maintenance. In a prior art U.S. Pat. No. 4,360,756, issued Nov. 23, 1982 to Spencer et al and assigned to the present assignee, a composition of fill materials is described and claimed which results in a reduction of the free iodine within the arc tube in the arc tube atmosphere during lamp operation to maintain a layer of thorium on the electrode tip during operation. Spencer et al describe the addition of getters (e.g. cadmium and zinc) to limit the build-up of iodine within the arc tube and thereby provide an enhancement of the thorium transport cycle.